Writing instruments having multiple writing elements are well known in the art. For the most part, these writing instruments have a plurality of writing elements disposed adjacent (side-by-side) to one another within a pen barrel. Prior art writing instruments have combined a variety of types of writing elements, including ball point pens, highlighters, and markers, in various combinations (e.g., all the same type, one of each, or more than one of more than one type). A drive mechanism is actuated to displace the writing tip of one of the writing elements to a position outside the barrel. In some cases, one writing instrument is already in a fixed position outside the barrel and the second writing element is moveable.
In order for the moveable writing element to be used for writing, it must be projected out of the barrel past the distal-most portion of the fixed writing element. In other known writing instruments, the distal-most portion of the moveable writing element—when fully projected—is in the same plane as the distal-most portion of the fixed writing element. Therefore, the two writing elements can produce two lines, or a line with a thickness greater than either writing element individually. Also, if the writing elements are supplied by two different writing mediums, two different lines can be produced.
The disadvantage of side-by-side construction is that the diameter of the pen barrel housing the writing elements must be greater than the diameter of a standard pen having only one writing element. To create a more compact writing implement, prior art devices have mounted the writing elements coaxially (i.e., one writing element disposed within another writing element), whereby the inner writing element is moveable relative to the outer writing element.
Various compact, multiple-writing-element writing instruments currently exist. These instruments have several distinguishing features. For example, U.S. Pat. No. 5,026,189, filed on Apr. 5, 1990, and issued to George Keil on Jun. 25, 1991, discloses a writing instrument having a pen barrel with two writing elements coaxially mounted therein. The inner writing element moves axially relative to the outer writing element. In one embodiment, each writing element has its own ink reservoir. The driving mechanism for moving the inner writing element relative to the outer writing element, however, is located towards the center of the pen barrel. Consequently, the writing tips must be separated from their respective ink reservoirs. The construction of such a writing instrument thus is complex and difficult to assemble en mass. Other prior art devices have inner and outer writing elements, which share a common ink reservoir. Such a configuration is undesirable if an operator wants to use different types of writing mediums.